Take a hand full of your gravel (trying for a good mix of the types of stone there) and drop it in a jar of vinegar. If it fizzes big time, then you may be in trouble. However seeing as you say your pH is popping back up to only the mid 7 range, that gives me some hope that you are not dealing with major limestone and perhaps just some residual calcium carbonate buffering from your water or dust from the rocks. Most really problematic limestones will buffer over 8. I've got shells in my system and I'm lucky they only buffer to 7.6 so I am still able to grow many things in my big aquaponics system.
So, with my hard well water and starting up my 300 gallon system which I made sure had no shells in it, It took me about 3 doses of sulfuric acid to get the pH down to 7.6 from over 8 which my well water is buffered to coming from a limestone aquifer and then while fishless cycling the pH did then drop below 7 and it then required me to add some calcium carbonate back in. During the dry season here my well water is easily able to buffer my system pH up over 7.6 when the biofilter is not being heavily pushed by fish load. During the rainy season (since my systems are out under the sky) my pH will come down and could even require buffering in the systems without shells.
Hope this example helps. I expect you just need to finish cycling up, patience. You say you put the plants in a week ago but you are worried that it isn't cycling up quick enough? When did you start the fishless cycling process? Even though it may happen quicker in some situations, cycling can take 6 weeks. Please don't start freaking out at week two because even those who talking about cycling up really quick, still allow 3 weeks before the spikes come down under optimum conditions.
Hang in there.
Just visited your blog. Nice setup. I enjoyed feeding the fish. THEY JUST KEPT EATING AND EATING.
Thanks for sharing.
What is the metallic material that is wrapped around your tank and grow bed? Is that provide insulation?
Your blog photos are great. I really like your approach to setting up your system. This again shows how versitile one can make an AP system.
the wrapping is an aluminum bubble wrap.. kinda expensive but worth it for the longevity of the system
Are those all IBC Totes wrapped in that stuff? It is not only functional, but a very clean / professional look.